The Krasnapolski is a top-of-the-line hotel in Amsterdam, the Netherlands. Among their many services, they rent bicycles to guests. The bicycle checkout is open 24 hours per day 7 days per week and has 50 bicycles on hand. On average, 10 guests request a bicycle each day, arriving completely randomly at all times of the day during the spring and summer. Guests keep bicycles for four days on average, with a standard deviation of two days. How many guests are waiting for a bicycle on average?